1. Field of the Invention
The present invention relates in general to magnetic resonance tomography (MRT) as used in medicine for examining patients. The present invention relates in particular to a method for preventing the ambiguity artifact, particularly when using spin-echo sequences as well as when using gradient echo sequences, but without affecting the measurement duration and the signal-to-noise ratio.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging technique for more than 15 years in medicine and in biophysics. In this examination method, the subject is exposed to a strong, constant magnetic field. As a result, the nuclear spins of the atoms in the subject become aligned after having been previously randomly oriented. Radio-frequency energy can now excite these “ordered” nuclear spins to product a certain oscillation. In MRT, this oscillation produces the actual measurement signal which is detected using suitable receiving coils. By using non-homogeneous magnetic fields produced by gradient coils, the examination subject can be spatially coded in all three spatial directions in the respective region of interest (also known as the field of view or FOV for short), this procedure being generally known as “spatial coding.”
The recording of data in MRT takes place in what is known as “k-space” (frequency domain). The MRT image in the “image” is linked thereto using a Fourier transformation with the MRT data in k-space. The spatial coding of the subject which defines the k-space takes place using gradients in all three spatial directions. The slice selection (gradient defines slice in the subject from which data will be acquired, normally along the Z axis), the frequency coding (gradient defines a direction in the slice, normally along the x axis) and the phase-coding (gradient defines the second dimension within the slice, normally the along y axis).
In other words, first a slice is excited selectively, for example, in the z direction. The coding of the spatial information in the slice takes place by combined phase and frequency coding by means of these two orthogonal gradient fields already mentioned which are produced in the example of a slice excited in the z direction by the aforementioned gradient coils in the x and y directions.
A first possible form of recording the data in an MRT exposure (measurement) is shown in FIGS. 2A and 2B. The sequence used is the known spin-echo sequence. The magnetization of the spins is flipped in the x-y plane by a 90° excitation pulse. Over the course of time, a dephasing of the magnetization components occurs, which jointly form the cross magnetization in the x-y plane Mxy. After a certain time (e.g., ½ TE, where TE is the echo time), a 180° pulse is emitted in the x-y plane so that the dephased magnetization components are mirrored without changing the precession direction and precession speed of the individual magnetization components. After a further time interval of ½ TE, the magnetization components again point in the same direction, i.e., a regeneration of the cross magnetization (known as “rephasing”) occurs, and the magnetic resonance signal is read out. The complete regeneration of the cross magnetization is known as “spin-echo”.
In order to measure an entire slice of the examination subject, the imaging sequence is repeated N times for different values of the phase-coding gradient, e.g., Gy, the frequency of the magnetic resonance signal (spin-echo signal) being scanned, digitized and stored for each sequence pass through the Δt-clocked ADC (analog/digital converter) N times in equidistant time steps Δt in the presence of the readout gradient Gx. In this manner, one obtains as shown in FIG. 2B a numerical matrix produced row-by-row (matrix in the k-space or k-matrix) having N×N data points (a symmetrical matrix with N×N points is only an example; asymmetrical matrices can also be produced). From this data set, using a Fourier transformation an MR image of the observed slice can be directly reconstructed with a resolution of N×N pixels.
The scanning of the k-matrix (k-matrices in the case of data from multiple slices) typically requires, for spin-echo sequences with diagnostically usable image quality, several minutes of measurement time, which is a problem in many clinical applications. For example, patients cannot remain still for the required interval of time. In the case of examinations in the thorax or pelvic regions, movement of the anatomy is generally unavoidable (heart and respiratory movement, peristalsis). A way of speeding up spin-echo sequences was published in 1986 as the turbo spin-echo sequence (TSE sequence) or rather under the acronym RARE (Rapid Acquisition with Relaxation Enhancement) (J. Hennig et al., Magn. Reson. Med. 3, 823–833, 1986). In this imaging technique (which is much faster than the conventional spin-echo technique described above), after a 90° excitation pulse a plurality of multiple echoes are generated, each of these echoes being individually phase-coded. A corresponding sequence diagram is shown in FIG. 3A for the case in which respectively seven echoes are produced. Before and after each echo, the phase-coding gradient must be switched corresponding to the Fourier row to be selected. In this manner, after a single RF excitation pulse (90°) a row-by-row scanning of the k-matrix takes place, as is shown in FIG. 3B. The required overall measurement time is reduced in this example by a factor of seven. The signal function is shown in FIG. 3A in an idealized fashion. In reality, the later echoes have smaller and smaller amplitudes due to the decay of the cross magnetization T2.
An even faster imaging sequence is represented by a combination of RARE with Half-Fourier technology; this was presented in 1994 and is known as the HASTE sequence (Half-Fourier Acquired Single shot Turbo spin Echo) (B. Kiefer et al., J. Magn. Reson. Imaging, 4(P), 86, 1994). HASTE uses the same basic technology as RARE, but only a half of the k-matrix is scanned. The other half of the k-matrix is reconstructed computationally using a Half-Fourier algorithm. Here, one takes advantage of the fact that the data points of the k-matrix are arranged with mirror symmetry about the center point of the k-matrix. For this reason, it is sufficient to measure only the data points of a k-matrix half and then to computationally fill out the raw data matrix by mirroring with respect to the center point (and complex conjugation). In this manner, the measurement time can be reduced by half, but the reduction of the recording time is associated with a degradation of the signal-to-noise (S/N) ratio by a factor of √2.
A further method of obtaining or scanning the k-matrix in a quick manner is a technique known as “gradient echo imaging” (GE imaging, GE sequence). The pulse and gradient pattern of a typical GE sequence is shown schematically in FIG. 4. As with the spin-echo sequences, here as well a rephasing in terms of the slice selection gradient Gz takes place and a pre-dephasing in terms of the frequency coding gradient Gy. Due to this gradient switching, the dephasing of the cross magnetization caused by the gradients is compensated so that an echo signal arises which is known as “gradient echo” (GE). Sequences in which the echo signal is generated exclusively through gradient inversion are known as gradient echo sequences (GE sequences).
In contrast to spin-echo sequences, the nomenclature for GE sequences is not consistent, and varies from company to company. The two most common GE sequences are known as the FLASH sequence (“Fast Low Angle SHot”) and the FISP sequence (“Fast Imaging with Steady Precession”). The two differ only in that the cross magnetization in the FLASH sequence is spoiled after data acquisition (using a spoiler gradient) (“spoiled GE sequence”) whereas it is maximized in the FISP sequence (“refocused GE sequence”). The GE sequence in FIG. 4 thus represents a FLASH sequence with the spoiler gradient.
In MRT imaging using spin-echo sequences as well as when using GE sequences, there is the general problem that the resonance condition during the radio-frequency excitation by the HF pulse exists not only in the field of view (FOV, characterized by homogeneity of the basic field as well as the gradient fields) but also in the non-homogeneous boundary region of the FOV. This means that superimposed on the image of the actual measurement field is a generally disruptive image from the inhomogeneity region in the form of an artifact. This undesired artifact is known as an “ambiguity” artifact. The ambiguity artifact becomes more pronounced the shorter the extent of the basic field magnet in the z direction.
Heretofore, the ambiguity artifact could be suppressed only in spin-echo sequences but not in gradient echo sequences. U.S. Patent Application Publication No. 2002/0101237 describes suppression thereof (unlike the conventional slice excitation of an spin-echo sequence) by switching the slice selection gradient during the slice excitation by the (90°) RF pulse, in comparison to the slice selection gradient which is switched during the (180°) refocusing pulse, with inversion in terms of its sign (polarity). This results in the resonance condition of the 90° pulse being fulfilled outside of the FOV at a different spatial position from the resonance condition when using the 180° pulse with the inverted sign. The ambiguity artifact cannot arise because the magnetization excited outside of the FOV is not refocused. A technique for suppressing the ambiguity artifacts in GE sequences is currently not known.